Field of the Invention
The present invention relates to a thin film transistor substrate used in a display or the like and to a method for manufacturing the same.
Description of the Background Art
Active matrix substrates (hereinafter referred to as “TFT substrates”) including thin film transistors (hereinafter referred to as “TFTs”) as switching elements are widely known to be used in electro-optic devices, such as displays including liquid crystals (hereinafter referred to as “liquid crystal displays; LCDs”).
The LCDs including the TFT substrates (TFT-LCDs) need improved display characteristics (such as a wide viewing angle, high definition, and high quality) and also need low costs as a result of simplifying manufacturing steps for an efficient manufacturing.
The TFT-LCD typically has a liquid crystal cell as a basic structure in which a TFT substrate (element substrate) and a counter substrate (CF substrate) sandwich a liquid crystal layer, and the LCD is formed of the liquid crystal layer including a polarizer bonded thereto. The TFT substrate includes a plurality of pixels arranged in matrix, each of the pixels including a pixel electrode and a TFT connected to the pixel electrode. The counter substrate includes a counter electrode disposed opposite to the pixel electrode and a color filter (CF). For example, a full transmissive LCD includes a backlight (BL) on a back surface side of the liquid crystal cell.
In this manner, the liquid crystal cell including the pixel electrode and the counter electrode disposed to sandwich the liquid crystal layer is a liquid crystal cell of a vertical electric field driving mode typified by a twisted nematic (TN) mode, the pixel electrode and the counter electrode generating an electric field for driving the liquid crystals. The TFT substrate of the TN mode is typically manufactured in four or five photolithography processes, as disclosed in Japanese Patent Application Laid-Open No. 10-268353 (1998) and Japanese Patent Application Laid-Open No. 2001-339072, for example.
Meanwhile, in terms of the wide viewing angle of the TFT-LCD, an in-plane switching (IPS) mode (“IPS” is a trademark) has been developed, the IPS mode being one of lateral electric field driving modes in which both of the pixel electrode and the counter electrode (common electrode) are disposed on the same layer on the TFT substrate. The IPS mode can obtain the viewing angle wider than that in the vertical electric field driving mode, but hardly obtains bright display characteristics due to an aperture ratio and a transmittance of an image display portion lower than those in the vertical electric field driving mode. This problem arises in a situation where an electric field for driving liquid crystals fails to effectively operate on the liquid crystals in a region directly above the pixel electrode having a comb teeth shape.
A fringe field switching (FFS) mode has been developed as a lateral electric field driving mode capable of improving this problem, as disclosed in Japanese Patent Application Laid-Open No. 2001-56474, for example.
Moreover, in terms of the high definition and the high quality of the TFT-LCD, technologies have been developed to use a semiconductor made of an oxide material having a mobility higher than that of conventional silicon (Si) as a semiconductor for an active layer of the TFT formed in the TFT substrate, as disclosed in Japanese Patent Application Laid-Open No. 2004-103957, Japanese Patent Application Laid-Open No. 2005-77822, and Kenji Nomura et al (2004), “Room-temperature Fabrication of Transparent Flexible Thin-film Transistors Using Amorphous Oxide Semiconductors”, Nature, 432: 488-492.
The oxide semiconductor includes zinc oxide (ZnO) system, InGaZnO system in which gallium oxide (Ga2O3) and indium oxide (In2O3) are added to zinc oxide (ZnO), or the like. Such oxide semiconductor has a film having light-transmissive properties higher than those of a Si semiconductor film, and Japanese Patent Application Laid-Open No. 2007-115902, for example, discloses that an oxide semiconductor film having a transmittance of 70% or more to visible light with wavelengths of 400 to 800 nm is used. Furthermore, Japanese Patent Application Laid-Open No. 2013-254121 discloses a liquid crystal display that makes use of the light-transmissive properties of the oxide semiconductor and uses part of one oxide semiconductor pattern as a channel semiconductor of a TFT and the other part thereof as a transparent pixel electrode by increasing an electrical conductivity by irradiation with an energy ray.
The typical oxide semiconductor film mentioned above can be etched with a weak acid solution, such as oxalic acid and carboxylic acid, and has an advantage of an easy patterning process. However, it is easily dissolved by an acid solution normally used for an etching process of typical metal films (Cr, Ti, Mo, Ta, Al, Cu, and alloys thereof) that are used for a source electrode and a drain electrode of the TFT.
Thus, the oxide semiconductor film needs not to be lost upon etching (patterning) of the metal films that are to be the source electrode and the drain electrode in a process of manufacturing them, depending on a structure of the TFT substrate.
For example, Japanese Patent Application Laid-Open No. 2008-72011 discloses a technology of adding new elements to an oxide semiconductor to improve resistance to a chemical solution and a technology of optimizing film thicknesses of the metal films that are to be the source electrode and the drain electrode and a film thickness of the oxide semiconductor. However, it is desirable to manufacture the TFT substrate using the typical oxide semiconductor film without using the technology of improving the resistance to the chemical solution.
There is an increased demand for the LCD of the FFS mode because of its sufficient viewing angle characteristics and its transmittance of a panel. However, the TFT substrate used in the LCD of the FFS mode needs to include both of the pixel electrode and the counter electrode (common electrode) formed on the TFT substrate, thereby increasing the number of wiring layers of the TFT substrate. This increases the number of photolithography processes needed for forming the TFT substrate, thereby increasing manufacturing costs.
For example, the TFT substrate in the typical LCD of the FFS mode disclosed in FIGS. 1 and 3 in Japanese Patent Application Laid-Open No. 2001-56474 is manufactured in six photolithography processes. On the other hand, the TFT substrate of the conventional TN mode can be typically manufactured in four or five photolithography processes, as disclosed in Japanese Patent Application Laid-Open No. 10-268353 (1998) and Japanese Patent Application Laid-Open No. 2001-339072. Thus, reducing the number of photolithography processes is a major challenge in manufacturing the TFT substrate of the FFS mode. If it is made possible to reduce more photolithography processes also in the TFT substrate of the conventional TN mode and to manufacture the TFT substrate in, for example, three photolithography processes, the manufacturing costs can be reduced, which is certainly more desirable.
As described above, the typical oxide semiconductor film is easily dissolved by the acid solution used for etching the metal films (Cr, Ti, Mo, Ta, Al, Cu, and alloys thereof) that are used for the source electrode and the drain electrode of the TFT. Therefore, for a structure that exposes a lower layer of a semiconductor film upon the etching process of the source electrode and the drain electrode, such as the structure of the TFT shown in Japanese Patent Application Laid-Open No. 2001-56474 (FIGS. 1 and 3), a special oxide semiconductor having the resistance to the chemical solution needs to be used, resulting in difficulty in using the typical oxide semiconductor.
Furthermore, when the oxide semiconductor film is used for a channel layer of the semiconductor, it is difficult for the structure of the TFT, such as the structure in Japanese Patent Application Laid-Open No. 2001-56474 (FIGS. 1 and 3), to stably obtain sufficient electrical characteristics of an interface at an electrical bonding portion between the source electrode and the drain electrode, and the oxide semiconductor having the high mobility fails to make full use of its performances due to an increase in electrical resistance at the interface.
Moreover, the liquid crystal display in Japanese Patent Application Laid-Open No. 2013-254121 needs a special process not being used for manufacturing the typical liquid crystal display such that the oxide semiconductor is irradiated with the energy ray to be used as the pixel electrode, thereby increasing the manufacturing costs. It is known that an electrical conductivity temporarily increases and then decreases due to a change over time or a heat treatment in the technique of decreasing a resistance (increasing a carrier) by the irradiation with the energy ray to increase the electrical conductivity, so that necessary electrical characteristics cannot be obtained in the end. Additionally, in the liquid crystal display in Japanese Patent Application Laid-Open No. 2013-254121, the channel semiconductor of the TFT and the transparent pixel electrode are separately formed with an end of the gate electrode disposed in the lower layer as a boundary therebetween, and thus the gate electrode fails to sufficiently shield the channel semiconductor from the light, thereby deteriorating the electrical characteristics.